(1) Field of the Invention
The present invention relates to a novel and highly effective process of recovering, separating, and concentrating citric acid and/or isocitric acid and their salts from aqueous solutions and more particularly from fermentation solutions, thereby separating said acids and their salts from other organic compounds and inorganic salts.
(2) Description of the Prior Art
For solving many problems of separating compounds from each other, there have proved to be especially effective separation processes which utilize a pressure gradient as effective force. Such processes make use of porous and/or semipermeable membranes, as used, for instance, in ultrafiltration and reverse osmosis. There are also known, for instance, processes for reclaiming and purifying certain specific waste waters or sewage, for desalinifying seawater, as well as a number of industrially performed processes for concentrating, isolating, or purifying various types of different compounds. See, for instance, "CZ-CHEMIETECHNIK" vol. 2 (1973), pp. 7 - 11.
By ultrafiltration there are separated at an operating pressure of 7 bars preferably colloidal particles especially according to their particle size (of a diameter between 10.degree. and 10.sup.2 nm) by means of microporous membranes. In contrast thereto hyperfiltration cannot be explained as a simple filtration process. The principle of hyperfiltration which today is generally accepted and which is characterized by the synonym "reverse osmosis" is the reversal of the tendency to cause osmotic dilution on semi-permeable membranes by means of a pressure difference. In this case at least the osmotic pressure difference (.DELTA..pi.) is to be overcome. The model of solution and diffusion as it has been described by H. Yasuda and C. E. Lamaze, Office of Saline Water, "Research and Development Progress Report" No. 473, September 1969, has become generally accepted as a theoretical basis for the transport mechanism and thus also as a basis for the membrane concept. This model takes into account more particularly the correlation between the molecular structure of the polymer membrane material and its separating behavior or effect
The best known semi-permeable membranes as they are used most frequently in actual operation at the present time consist of cellulose derivatives. Recently, however, other plastic materials such as, for instance, polyacrylic or, respectively, polymethacrylic derivatives, polyamides, and the like, have been employed as membrane material. In these cases a distinction is made between homogeneous and asymmetric membranes in accordance with membrane morphology. The general trend in connection with separation processes using membranes aims at investigations to produce membranes of optimum selectivity for a specific predetermined separation problem.
Thus membranes for pervaporation which are suitable for selective separation of water from aqueous starting mixtures with organic and inorganic dissolved constituents, are known. They consist of an organic polymer having anionic groups which may also contain cationic groups. But according to the inventions to be mentioned in this connection by example (see, for instance, German Published applications Nos. 2,129,723 and 2,129,734), the use of a liquid phase on both sides of the membrane is impracticable because such membranes require for this purpose pressures up to 1,000 bars. Inorganic salts are not to be separated in this manner in accordance with this problem. Thus such membranes are useless for solving the problem upon which the present invention is based.
Selectively permeable membranes are described in German Published application No. 2,051,631. Such membranes consist of copolymers which contain at least 50% of hydrophilic, non-ionic groups, above all hydroxyl groups while the remainder comprises basic or carboxyl groups-- containing units. These copolymers are, as stated, more selective when used as ion exchange membranes, as proved on the example of sodium ions Na.sup.+ and calcium ions Ca.sup.++. Data regarding the selectivity properties of these membranes in relation to mixtures of polycarboxylic acids and/or their salts and inorganic monovalent or, respectively, polyvalent anions, however, are not given in the published application.
Membranes with selective permeability which consist of vinyl copolymers and which are synthesized from three or more monomers, are mentioned in German Published application No. 2,044,509. Cross-linking with known bifunctional monomers is also mentioned therein. The selectivity of such membranes, however, has been proved only for the system sodium chloride -- water.
German Published application No. 2,027,698 discloses a process for purifying aqueous itaconic acid solutions by reverse osmosis. Polyamides and cellulose esters are listed as membrane materials. According to this application working up a citric acid solution, in place of the itaconic acid solution, has also been tried. However, it was found that the selectivity in this case is very unsatisfactory and that the membranes are not suitable for this purpose.